I need help designing a hardwired circuit that will alert me when my dog goes in and out the pet door, I'm disabled and 80% deaf so an audible device won't work, lol! I want to use a magnetic door contact from a home alarm system, an LED, a plug-in power supply and a relay to send power to the LED when the pet door is open, it does not need to be a latching circuit because I want the LED to go off when the pet door closes! LED on when open, LED off when closed! Sounds simple enough, can anyone help me?

Do you have any wallwarts available (plug in power supply)? and what output voltage? This will help in deciding what component values to use with the LED..... and you do not necessarily need a relay if you use a standard LED, since these normally do not draw too much current and a Reed Relay (Alarm contacts you mentioned) can handle an LED as a load. So if you have a power supply already handy, then we could build a circuit around that.....

Match a W-W,wall-wart or AC adapter output voltage to an = V SPDT relay, 5V to 12V. Relay coil resistance should be as high as you can find, 700Ω or better. Mount magnet so that reed switch is closed when door is closed. Need to know LED color , operating current & voltage to calculate R.